1. Field of the Invention
The present invention relates generally to a pressure-sensitive, conductive elastic sheet and particularly to a pressure-sensitive, conductive elastic sheet sandwiched between a pair of flat electrode plates, each having a group of straight strip electrodes which is used as a graphics digitizing tablet through which various letters or figures can be detected two-dimensionally by a character reader.
2. Description of the Prior Art
Pressure-sensitive, conductive elastic sheets are well-known. Elastic sheets of this kind is usually used in graphics digitizing tablets, which detect various letters or figures written thereon with a pen or the like. Such sheets are sandwiched between a pair of flat electrode plates provided with a group of straight strip electrodes in such a way that the strip electrodes on one flat electrode plate intersect those on the other flat electrode plate to form a matrix of points. In such a graphics digitizing tablet when pressure is applied to the pressure-sensitive conductive sheet via the upper and lower flat electrode plates, the conductive sheet makes electrical contact at points where pressure is applied by a pen and the contact points are digitized by the two perpendicular strip electrodes on the basis of co-ordinates. The surface area of the tablet on which letters or figures are written is relatively large, ranging from 100 mm square to 400 mm square. Accordingly, when some letters or figures are written on the tablet with a pen held in the user's hand, the heel of user's hand or the user's forarm inevitably depresses the surface of the tablet, with the result that erroneous operation often occurs in detecting the letters or figures. In this connection, experiment indicates that contact pressure (under which some of the upper strip electrodes are connected electrically to some of the lower strip electrodes) conventionally decreases with increasing contact area (throughout which some of the upper strip electrodes are connected electrically to some of the lower strip electrodes).
In other words, the upper and lower strip electrodes are easily connected electrically when light pressure is applied over a large conduction area on the tablet. To explain in more detail, when a pen is placed onto the tablet to write some letters, since the contact area is small, a relatively great contact pressure is needed to connect the upper and lower strip electrodes; on the other hand when the heel of the user's hand or the user's forearm is placed on the tablet, since the contact area is large, a relatively small contact pressure easily connects the upper and lower strip electrodes, thus resulting in erroneous operation.
A more detailed description of the prior-art pressure-sensitive, conductive elastic sheet for use in digitizing tablets will be described hereinafter with reference to the attached drawings in conjunction with the present invention under DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS.